Support extending buffer size with sd card

[jmadden173]

Describe the Enhancement
In 2.2.1 of the hardware a sd card slot will be added to the esp32. Integrating the sd card into the buffer would give much more space to sustain a network outage where the device cannot be uploaded.

Requirements

• Preserve order of measurements
• Upload all stored data when the network comes back, even after the device reboots

Benefits
More storage would allow for the device to continue measuring for a longer period of time.

Possible Drawbacks
Increased power consumption

Additional Context
Images showing software flow and the memory structure of the buffer